16.1 Pathogens of insects

Arthropods are the most diverse group of animals on Earth; insects occurring
in most terrestrial environments, though only a few species are found in marine
habitats, as these are dominated by the crustaceans. Estimates of the number of
arthropod species vary between 1,170,000 and 10 million, accounting for over 80%
of all known living animal species; with the insects as the most species-rich
subgroup in land and freshwater environments.

As arthropods were emerging as the (numerically) dominant animals they
are today, fungi were also colonising the land. Over the past 400 million
years or so, fungi and insects have evolved together in a wide array of
intimate associations; including mutualistic endosymbiosis; using fungi as
obligate food sources, such as those found in fungus-gardening ants (Section
15.2); sexually and behaviourally transmitted parasites, such as
Laboulbeniales (Section 16.4); and
with fungi as the most common disease-causing agents of insects, and many
entomologists believe that there may be more species of insect
pathogens than there are species of insects (Lovett & St.
Leger, 2017).

Insect species are infected by pathogenic viruses, bacteria, and protozoa, as
well as two groups of organisms that have uncertain relationships, the
microsporidia and the trichomycetes; all of these tend to cause infection after
being ingested by the insect.

The true fungi that infect insects are invasive pathogens;
that is, they can produce enzymes and hyphae that can penetrate the insect
cuticle. These disease organisms often act as natural control agents by
regulating the population size of insect pests. There is consequently
considerable interest in harnessing this capability commercially to control
insects that are pests because of their adverse effects on our agricultural
activities or because they act as vectors of human diseases (Lacey et al.,
2015; Butt et al., 2016; and see chapters 1 & 2 in Ecofriendly Pest
Management for Food Security, Omkar, 2016; Blackwell, 2017; Lovett & St.
Leger, 2017).